Upon triggering, many long metal spears pop up from the floor and out from the wall simultaneously, and lock into place. Anyone in the area may be struck by 1d4 spears, taking 1d6 HP damage from each. AC and/or Dex rolls/Saving throws to avoid, may apply, as might acrobatic skills, etc.
Anyone struck by a spear is pinned in place, unable to move. Successful strength rolls may or may not be able to bend the bars enough to get free. Magic, specialized metalworking tools, or other things may also be of help.
Optional: once this trap is activated, the area may become impassable.
Metal Spears Over Yellow created with Inkscape by David